1. Field of the Invention
The present invention relates to a plasma processing system for sputter deposition applications, and more particularly, to a plasma processing system like a plasma assisted sputtering system having an improved plasma source capable of independently controlling plasma ion density and ion energy at the rf (high-frequency AC) electrodes useful for a sputtering process of metal or dielectric materials during the fabrication of integrated circuits in the semiconductor industry.
2. Description of the Related Art
Large-area and high-density plasma sources with higher radial uniformity are of great demand to process large area substrates without charge-induced damages to the devices fabricated on the substrate surface. Specially, development of new plasma sources for the sputtering process of metal and dielectric materials with enhanced uniformity of the deposited film is of important. Difficulties in obtaining the above-mentioned properties with conventional plasma sources are explained using two conventional configurations as shown in FIGS. 10-13, which are usually applied in 200 mm wafer or flat panel plasma processing systems.
FIG. 10 shows a simplified conventional magnetron-type plasma source that uses for sputter deposition applications in semiconductor industry. A reactor 101 is comprised of an upper electrode 102 made of a non-magnetic metal, a cylindrical side wall 103 and a lower electrode 104. The upper electrode 102 forms a top plate of the reactor 101 and the lower electrode 104 is arranged on a bottom plate 105 of the reactor 101. The upper electrode 102 and the lower electrode 104 are parallel to each other across at least over a portion of the reactor 101. The side wall 103 and the bottom plate 105 are made of a metal, for example stainless steel. The upper part of the side wall 103 is made of an insulating material 106 on which the upper electrode 102 is placed. A target plate 107 made of a material needed to be sputtered is fixed to the lower surface of the upper electrode 102. Usually, the target plate 107 has slightly smaller dimensions in comparison with the upper electrode 102. On the upper surface of the upper electrode 102 as the top plate, two magnets 108a and 108b of circular and ring shapes are concentrically placed as shown in FIGS. 10 and 11. The central magnet 108a is of a cylindrical shape without any cavity as shown in FIG. 11. The outer magnet 108b is of a ring shape. The height and widths of each of the magnets 108a and 108b are not critical and selected according to the other dimensions of the reactor 101. The magnets 108a and 108b are placed on the upper electrode 102 so as to have opposite polarities facing the inside of the reactor 101. This arrangement of the magnets 108a and 108b generates curved magnetic fields 109 between these two magnets.
The upper electrode 102 is connected to a high-frequency AC (rf) electric power source 110 through a matching circuit 111. The frequency of the rf electric power source 110 is usually 13.56 MHz. When a rf electric power is applied to the upper electrode 102, plasma is generated by the capacitively-coupled mechanism. Once the plasma is made, electrons in the plasma are confined within the curved magnetic fields causing an increase of plasma density in that region.
A substrate 112 is placed on the lower electrode 104 electrically isolated from the bottom plate 105 through an insulating material 113. The lower electrode 104 may or may not be given a rf electric power from a rf power source. If a rf electric power is supplied to the lower electrode 104 by a rf electric power source 114 through a matching circuit 115, as shown in FIG. 10, the frequency of the rf electric power source 114 usually lies in MF region. When a rf current is applied to the lower electrode 104, it gets negatively biased causing an ion bombardment onto the surface of the substrate 112. Though the ion bombardment causes an etching process on films deposited on the substrate 112, the self-bias voltage of the lower electrode 104 is controlled so that the film deposition rate exceeds the film etching rate on the substrate 112.
Another conventional magnetron type sputtering source shown in FIG. 12 is a slight modification of the above-mentioned plasma source given in FIG. 10. Here the central magnet 108a is placed in an off-axis mode in order to form an asymmetric magnetic field below the upper electrode 102. A top view of this magnet arrangement is shown in FIG. 13. This magnetic configuration is rotated around a central axis (shown as a dashed line 116 in FIG. 12) of the upper electrode 102. The magnet arrangement formed by the magnets 108a and 108b shown in FIGS. 12 and 13 rotates asymmetrically.
The parallel plate plasma reactor shown in FIG. 10 has several advantages such as large area plasma between the parallel electrodes, readily ignition of the plasma, and the ability of controlling plasma ion energy at the lower electrode surface. With the magnet arrangement given in FIG. 10, a doughnut-shaped curved magnetic field is generated below the upper electrode 102. Once the plasma is ignited, higher-density plasma of the doughnut-shaped is formed below the upper electrode 102 due to the magnetic confinement of electrons. Since this higher-density plasma is mainly confined within the region between the magnetic poles of the magnets 108a and 108b, there is a lower plasma density in the vicinity of the magnetic. poles.
Further, the strength of the magnetic field increases toward the magnetic poles. This causes a mirror reflection of the electrons that result in lower electron density at the magnetic poles of the magnets 108a and 108b. When the electron density is low, the ion density is also gets low since ions are trapped in the plasma by electrostatic fields generated by electrons.
Because of the two reasons explained above, the ion flux at the magnetic poles gets smaller to result in a lower sputtering rate. However, since there is a higher-density plasma in the doughnut-shape region between the respective magnetic poles of the magnets 108a and 108b, the area of the target plate 107 corresponding to the region between the two magnets gets strongly sputtered. A fraction of these sputtered atoms are reflected back due to the scattering by gas molecules and deposited again on the target plate 107. Since the sputtering rate at the places of the target plate surface corresponding to the magnetic poles is relatively smaller, deposition of the sputtered atoms at these places gets dominant. The re-deposited film, however, has a lower density and stick loosely on the target plate 107, thus it can be easily released as particles.
In order to avoid the re-deposition of sputtered materials on the target plate 107, as shown in FIG. 12, the magnets 108a and 108b are placed asymmetrically and rotated around the central axis 116 of the upper electrode 102. Even though there is the re-deposition of sputtered materials at the places corresponding to the magnetic poles, the re-deposited films are immediately sputtered back into the plasma due to the rotation of the magnets. Accordingly, the source of particles in the plasma can be eliminated.
However, the plasma generated with the configuration given in FIG. 12 is radially non-uniform. This causes a non-uniform ion flux onto the surface of the substrate 112. This may cause localized charge build up on the substrate surface, specially if the substrate 112 is negatively biased by applying the rf electric power to the lower electrode 104, which eventually results in electrical breakdown of sub-micro scale elements on the substrate 107.
An object of the present invention is to provide a magnetically enhanced capacitively-coupled plasma processing system for sputter deposition applications with higher ion concentration, higher ion flux uniformity on the substrate surface and without the re-deposition of sputtered materials back on the target plate.
A plasma processing system of the present invention has the following structures in order to attain the above-mentioned object.
A plasma processing system for sputter deposition applications in accordance with the present invention has a reactor including parallel capacitively-coupled upper and lower electrodes facing each other across at least a portion of the reactor. A substrate to be processed is loaded on the lower electrode and a target plate to be sputtered by plasma is arranged at an inner (lower) side of the upper electrode. Further, the plasma processing system has high-frequency AC electric power sources for respectively supplying AC electric power to the upper electrode and/or the lower electrode. The AC electric power source for the upper electrode preferably operates at HF or VHF region. The AC electric power source for the lower electrode preferably operates at MF, HF or VHF region. Plural magnets are arranged radially in outer region of the upper electrode, and they are preferably rotated by a predetermined mechanism around a central axis of the upper electrode.
In the above-mentioned plasma processing system, preferably, an additional DC electric power source is connected to the upper electrode via a low-pass filter that cut off an AC current applied to the upper electrode.
In the above-mentioned plasma processing systems, preferably, the upper electrode is made of a non-magnetic metal, and the magnets generate magnetic fields with closed magnetic fluxes near to the inner surface of the upper electrode by changing a magnetic polarity of the magnets facing the inside of the reactor alternately.
In the above-mentioned plasma processing systems, preferably, other plural magnets are arranged along a circular line surrounding the radially-arranged magnets in order to confine electrons in a peripheral region of plasma.
In the above-mentioned plasma processing systems, preferably, the radially-arranged magnet with N porality facing the inside of reactor and the other magnet arranged along the circular line are lined up in series.
In the above-mentioned plasma processing systems, preferably, the magnets include first magnets with N polarity facing the inside of the reactor, which are of curved shape extending to the boundary of the upper electrode, and second magnets with S polarity facing the inside of the reactor, which are of straight shape, and the first magnets and the second magnets are alternately arranged so that electrons in the plasma within a magnetic line cusp between the first and second magnets are moved radially outward due to Exc3x97B drift and then curved and drifted radially inward through the magnetic line cusp.
In the above-mentioned plasma processing systems, preferably, the first magnet with either S or N polarity is longer than the second magnet with S polarity.
In the above-mentioned plasma processing systems, preferably, the magnets include first magnets with S polarity facing the inside of the reactor, which are of curved shape extending to the boundary of the upper electrode, and second magnets with N polarity facing the inside of the reactor, which are of straight shape, and the first magnets and the second magnets are alternately arranged so that electrons in the plasma within a magnetic line cusp between the first and second magnets are moved radially outward due to Exc3x97B drift and then curved and drifted radially inward through the magnetic line cusp.
In the above-mentioned plasma processing systems, preferably, the first magnet with S polarity is longer than the second magnet with N polarity.
In the above-mentioned plasma processing systems, preferably, the plural magnets are assembled on a circular metal ring to have a small separation between the upper electrode and the magnets.
In the above-mentioned plasma processing systems, preferably, the plural magnets are placed directly on the upper electrode.
In the above-mentioned plasma processing systems, preferably, each of the magnets is made as a single piece.
In the above-mentioned plasma processing systems, preferably, each of the magnets is consisted of several magnet elements.
In the above-mentioned plasma processing systems, preferably, width of each of the magnets existing on the radial line is varied in radial direction.